Compressors for Different Refrigerants
|NH3||Reciprocating & screw compressor|
Applications of Refrigerants
|NH3||Cold storage, ice plants, refrigerators|
|H2O||Water, Li Br absorption system|
|CO2||Used as dry ice in transport|
|R-11||Central air conditioning|
|R-12||Domestic refrigerator, water cooler|
|R-22||Window type air conditioner|
DESIGNATION OF REFRIGERANTS
- If the compound is CmHnFpClq
n + p + q = 2m + 2
designation is R(m - 1) (n + 1) P
Eg. CCI3 is R-11
- The brominated refrigerants are denoted by and additional B and a member to denote as to how many chlorine atoms are replaced by bromine atoms
Eg. CF3 Br is R13 B1
Unsaturated Compounds - CmHnFpClq
where n + p + q = 2m
they are designated by R (m-1) (n+1)P
Eg. C2H4 is R1150
Designation is R (700 + molecular weight)
Eg. NH3 is R 717, CO2 is R744
Basic properties of Some important Refrigerants
CFC 12 or R-12-or CCl2F2
- Normal boiling point (N.B.P.) = 29.8ºC
- It is used in domestic refrigerators, car air conditioners, water coolers etc.
- It is used with reciprocating compressor
- Because of low value of g. discharge temperature is low. so it is very suitable for hermetically sealed units
CFC-11 or R-11 or CCl3F
- N.B.P. -23.7ºC
- Having more boiling point, It has vacuum in the evaporator.
- It is used with centrifugal compressor.
- Its application is in central air conditioning.
HCFC-22 or R-22 or CHClF2
- N. B. P. – 40.8ºC
- It is used in window air conditioning.
- It is used with reciprocating compressor.
- It has high g value, so discharge temperature is high. Thus it is not very suitable for hermetically sealed unit for more than one year.
- N.B.P. of commercially used refrigerants vary from -50ºc to + 50ºC.
PROPERTIES OF GOOD REFRIGERANTS
- Low boiling point
- Evaporating pressure should be low and positive and near to atmospheric pressure
- High critical temperature
- Low critical pressure
- Low freezing point
- Non flammable, non explosive
- Either completely miscible or completely immiscible with oil
- Non corrosive
- High dielectric strength
- High thermal conductivity
- Low viscosity
- Low specific heat
- Low surface tension
- Low cost
- They are used to reduce the quantity of main refrigerant and to reduce pressure losses. They are cheap in cost.
Eg. brines such as : CaCl2 + Water & NaCl + water, Ethylene glycol + water and propylene glycol + water.
- CaCl2 and NaCl is mixed with water to reduce the freezing point of water.
- Ozone layer is in stratosphere.
- It protects humans from UV rays which can cause skin cancer.
- Chlorine atoms in CFC causes ozone depletion.
- Hydro-Chloro-flouro-carbons (HCFCs) have low ozone depletion tendency because of less chlorine.
But they have Global warming tendency.
- Pure hydro carbon is highly inflammable due to hydrogen atoms.
- Azerotrope: These are non ideal mixtures of two or more refrigerants behaving as pure substance having close N.B.P. Their bubble point temperature is equal to dew point temperature.
- Bubble temperature is the temperature at which liquid mixture starts boiling.
- Dew point temperature is the temperature at which vapour mixture starts condensing.
- CO2 refrigeration system has very low COP because critical temperature of CO2 is very low and very close to condensing temperature.
- Because of flammability & toxicity, ammonia is not used in domestic refrigerator, comfort air conditioning.
- If water content in refrigerant is more than that can be dissolved by the refrigerant, there is a danger of ice formation and consequently choking of capillary tube used in throttling system. This is moisture chocking.
- Silica gel is used in liquid line for dehydration of refrigerant.
- Miscibility of refrigerants with oil
|Refrigerant||Miscibility with oil|
|R-11, R-12||Good miscibility|
|NH3, CO2||No miscibility|
- In evaporator, oil separates from refrigerant vapour. Accumulation of oil in evaporator called oil choking.
- In immiscible oil refrigerant mixture, oil separator is used after compressor.
- Ammonia corrodes copper. So iron and steel are used with its system.
- Refrigerants in decreasing order of normal boiling point.
R-11(–23.7ºC) > R-12(-290C) > NH3(–33.30C) > R-22(–410C) > CO2(– 73.60C)
- Refrigerants in increasing order of freezing point
CO2 < NH3 < R-11 < R-12 < R-22
- Refrigerants in decreasing order of critical temperature
R-11 < NH3 < R-12 < R-22 < CO2
- Refrigerants in decreasing order of Critical Pressure
NH3(113.86) > CO2(73.8) > R-22 > R-11 > R-12(41.2)
- Refrigerants in decreasing of COP
R -11 > NH3> R-12> R-22> CO2
Leakage Detection of Refrigerants
|NH3||Burning sulphur candle|
|Halocarbon||Halide torch test|
|Hydrocarbon and Propane||Soap and water test|
|SO2||NH3 swab test|
|Refrigeration||electronic leak detector|